A vacuum-assisted, countergravity casting process using a gas permeable mold sealingly received in a vacuum housing is described in such patents as the Chandley et al U.S. Pat. Nos. 3,900,064; 4,340,108 and 4,606,396. That countergravity casting process involves providing a mold having a porous, gas permeable cope and a lower drag sealingly engaged at a parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable cope, submerging the underside of the drag in an underlying melt pool and evacuating the vacuum chamber sufficiently to draw the melt upwardly through one or more narrow ingates (pin gates) in the drag and into one or more mold cavities formed between the cope and the drag.
In practicing the vacuum-assisted, countergravity process to produce nodular iron castings, the melt is typically prepared in a melting vessel (e.g., a cupola) using a charge of pig iron to which additions of alloyants are made to provide the desired base melt chemistry. For example, in casting nodular iron, ferrosilicon (Fe-Si), ferromanganese (Fe-Mn) and other additions are made to the base pig iron charge to provide a desired base melt chemistry.
Once the desired base melt composition is achieved, the melt is transferred to a ladle where a nodularizing agent (e.g., a magnesium-bearing alloy such as Fe-Si-Mg) is added to spherodize (nodularize) the carbon in the melt. The treated base melt is then transferred from the ladle to a casting vessel to provide the melt pool from which a plurality of molds are successively vacuum-assisted, countergravity cast over time.
However, prior workers have experienced great difficulty in maintaining an effective concentration of magnesium in the melt over the extended time required to cast a plurality of molds in succession from the pool. This difficulty is attributable to the rapid evaporation of magnesium from the melt after the initial treatment with the nodularizing agent in the ladle. Erratic, uncontrolled loss (also known as fade) of the fugitive magnesium from the melt over time has been experienced and resulted in off-chemistry melts in so far as magnesium content is concerned and correspondingly inconsistent nodularization.
In the gravity casting of nodular iron, the mold has been provided with a reaction chamber and appropriate gating between the reaction chamber and the mold cavity to communicate them so as to fill the mold cavity with melt treated in the reaction chamber, e.g., as shown in the Duchenne U.S. Pat. No. 3,971,433. An appropriate quantity of nodularizing agent usually in particulate form is placed in the bottom of the reaction chamber so that the melt poured into the mold flows through the reaction chamber where it contacts the particulates in a manner to introduce the nodularizing agent therein to a sufficient extent to spherodize the carbon. This process is referred to in the casting art as the "in-mold" process.
This "in-mold" process has been applied to the vacuum-assisted, countergravity casting of nodular iron in that a reaction chamber has been formed in the countergravity casting mold beneath the mold cavity and a nodularizing agent in particulate form is placed in the bottom of the reaction chamber. The melt is drawn by a relative vacuum applied to the mold cavity to flow through the reaction chamber and then into the mold cavity from an underlying melt pool when a drag portion of the mold is immersed in the melt pool. The nodularizing agent is dissolved by contact with the melt in the reaction chamber and is thereby introduced into the melt to a sufficient extent to spherodize the carbon therein.
As applied to the gravity casting and vacuum-assisted, countergravity casting of nodular iron, the "in-mold" process is disadvantageous in that the metal gating weight of the resultant casting is increased as a result of solidified melt remaining in the reaction chamber and in the mold ingating that supplies melt to the mold cavity. Moreover, in countergravity casting thin-walled castings, the mold cavities fill so quickly that the reaction between the melt and the nodularizing agent does not occur to an effective degree to form an acceptable metallurgical microstructure. In addition, since the reaction products (e.g., slag, etc.) tend to float in the melt, they can contaminate the melt as it is drawn upwardly into the mold cavities, thereby yielding corresponding contaminated castings.
It is an object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein the melt is treated (e.g., nodularized) in a reaction chamber in the mold and drawn from the reaction chamber into a mold cavity while the mold/melt are engaged to partially fill the mold cavity and then further melt needed to complete filling of the mold cavity is drawn from the reaction chamber after mold/melt disengagement in a manner that removes melt from the reaction chamber so as to thereby reduce or minimize the gating weight of the resultant casting.
It is another object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein the melt is treated (e.g., nodularized) in a reaction chamber in the mold and is drawn into the mold cavity in a manner that provides time for the desired reaction of the melt and alloyant in the reaction chamber.
It is still another object of the present invention to provide an improved apparatus and method for the differential pressure, countergravity casting of a melt wherein the melt is treated (e.g., nodularized) in a reaction chamber in the mold and drawn into the mold cavity in a manner that reduces contamination of the casting and the underlying melt source by reaction products that may be generated in the reaction chamber.